1. Field of the Invention
This invention relates to an automatic focus adjusting apparatus for use in a camera or the like.
2. Related Background Art
A method of correcting the out-of-focus state attributable to the movement of a moving object when the moving object is always pursued by driving a lens by means of an auto focus apparatus has already been proposed by the same assignee in Japanese Patent Application No. 62-263728.
In the above-mentioned patent application, the movement of the imaging plane of the object is approximated by a quadratic function or a linear function and, on the other hand, the time required for distance measurement calculation, lens driving or release is foreseen under a certain assumption and the imaging plane position of the object at a certain time in the future (for example, the time when the lens driving control is completed, or the time when the shutter curtain is moved after the releasing operation) is foreseen, and in accordance with the result thereof, the lens is driven to said foreseen imaging plane position to thereby eliminate any pursuit delay relative to the object.
However, the movement of the object is continuous, whereas the focus adjusting operation is intermittent and therefore, there is the problem that depending on the timing of the releasing operation an out-of-focus situation occurs during photographing, i.e., during shutter opening. It will hereinafter be explained with reference to the accompanying drawings.
FIG. 2 is a graph for illustrating the above-described lens driving correction method.
In the figure, the horizontal axis represents time t, and the vertical axis represents the imaging plane position d of the object.
A curve f(t) represented by the solid line represents the imaging plane position at time t of the object coming near the camera in the direction of the optic axis when the photo-taking lens is at infinity. A curve l(t) represented by broken line means the imaging plane position of the object in the position of the photo-taking lens at time t, and a section [ti, ti'] is the focus detecting operation and a section [ti', ti+1] is the lens driving operation. Accordingly, the difference between f(t) and l(t) at the same time t in the direction of the vertical axis d corresponds to the so-called defocus amount.
DFi represents the defocus amount detected at time ti, DLi represents the amount of lens driving converted into the amount of movement of the imaging plane executed from the result of the focus detection at time ti, and TMi represents the time interval between the focus detecting operations.
In the example of the prior art shown in FIG. 2, as a premise for correction, it is assumed that the imaging plane position of the object changes in accordance with a quadratic function. That is, it is assumed that if the current and past three imaging plane positions (t.sub.1, f.sub.1), (t.sub.2, f.sub.2) and (t.sub.3, f.sub.3) are known at time t.sub.3, the imaging plane position f.sub.4 at time t.sub.4 can be foreseen.
However, what the camera can actually detect are not the imaging plane positions f.sub.1, f.sub.2 and f.sub.3, but the defocus amounts DF1, DF2 and DF3 and the amounts of lens driving DL1 and DL2 converted into the amounts of movement of the imaging plane. Time t.sub.4 is a value in the future, and actually is a value which varies with a variation in the accumulation time of an accumulation type sensor, caused by the brightness of the object, but here, for simplicity, it is assumed that t.sub.4 is known from the relation that t.sub.4 -t.sub.3 =t.sub.3 -t.sub.2.
Under the above-described assumption, the amount of lens driving DL3 when lens driving is effected at time t.sub.3, toward t.sub.4 from the result of the focus detection at time t.sub.3 is found by the use of the following equations: ##EQU1##
If the point l.sub.1 in FIG. 1 is considered to be the origin, ##EQU2##
If the equations (3) and (4) are substituted into the equations (2), (2)' and (2)" to find a, b and c, ##EQU3## Consequently, the amount of lens driving DL3 converted into the amount of movement of the imaging plane at time t.sub.4 is ##EQU4##
Here, assuming that as previously described, TM3 is known from the relation that TM3=TM2, DL3 can be found from the equation (8).
The amount of lens driving at time t.sub.n after time t.sub.4 can likewise be found from the past three detected defocus amount DF.sub.n-2, DF.sub.n-1, DF.sub.n, the past two actual amounts of lens driving DL.sub.n-2, DL.sub.n-1 and the past two time intervals TM.sub.n-2, TM.sub.n-1. ##EQU5## If in accordance with the equations (8), (9) and (10), the defocus amount DL.sub.n for effecting lens driving is found from the detected defocus amount DF.sub.n and lens driving is effected, proper focusing even to a moving object will always become possible at the end of lens driving.
Now, the aforementioned problem when the releasing operation takes place during such automatic focus adjusting control will be explained with reference to FIGS. 3 and 4.
FIG. 3 shows a case where under a situation in which focus detection is started at time t.sub.n and lens driving of DL.sub.n is effected at t'.sub.n and lens driving is completed at t.sub.n+1, the releasing operation has taken place at time t.sub.x1. Here, the time from after the releasing operation has taken place until film exposure is actually effected, i.e., the so-called release time-lag, is TR. Thus, in the figure, film exposure is effected at time t.sub.x1 +TR. In the case of such control that lens driving is stopped simultaneously with the releasing operation, the imaging plane position l.sub.x1 of the lens at time t.sub.x1 is intactly the imaging plane position l.sub.r1 of the lens at time t.sub.x1 +TR, and at this time, the imaging plane of the object lies at f.sub.r1 and thus, the object image exposed on the film suffers from defocus of f.sub.r1 -l.sub.r1 =d.sub.x1, i.e., out-of-focus.
In the case where that lens driving is continued even if the releasing operation takes place, the imaging plane position of the lens at time t.sub.x1 +TR is l'.sub.r1 and although small in amount, an out-of-focus situation of f.sub.r1 -l'.sub.r1 =d'.sub.x1 still occurs.
FIG. 4 shows a case where the releasing operation has taken place during lens driving. An in the case of FIG. 3, in such control that lens driving is stopped simultaneously with the releasing operation, an out-of-focus situation of f.sub.r2 -l.sub.r2 =d.sub.x2 occurs, and when lens driving is terminated simultaneously with the releasing operation, an out-of-focus situation of f.sub.r2 -l'.sub.r2 =d'.sub.x2 occurs.
A description will now be given of a correction method which takes a uniform release time-lag into account. In this case, the time t.sub.n+1 can be considered to extend by the release time-lag TR and therefore, the equation (10) is deformed as follows: EQU DL.sub.n =a.sub.n .multidot.[(TM.sub.n-2 +TM.sub.n-1 +TM.sub.n +TR).sup.2 -(TM.sub.n-2 +TM.sub.n-1).sup.2 ]+b.sub.n .multidot.(TM.sub.n +TR)+DF.sub.n( 11)
FIG. 5 shows the control of the above equation (11). f'(t) represented by dot-and-dash line is the imaging plane position of the object taking a uniform release time-lag TR into account, and the lens can be controlled so as to be along this curve. Accoridngly, the object in the finder is always out of focus by the release time-lag. Assuming that as in FIG. 3, the releasing operation has taken place at time t.sub.x1, if lens driving is immediately stopped, the imaging plane position of the lens lies at l.sub.r1 at time t.sub.x1 +TR, and the actual imaging plane position of the object is f.sub.r1 and therefore, on out-of-focus situation of f.sub.r1 -l.sub.r1 =d.sub.x1 occurs. Also, if after the releasing operation, the lens is driven by an amount DL.sub.n foreseen at time t.sub.n and the lens driving at that time is terminated, out-of-focus of f.sub.r1 -l'.sub.r1 =d'.sub.x1 occurs. FIG. 6 shows a case where the releasing operation (time t.sub.x2) has taken place during lens driving (time t.sub.n' -t.sub.n+1), and if lens driving is stopped simultaneously with the releasing operation an out-of-focus situation of f.sub.r2 -l.sub.r2 =d.sub.x2 occurs, and if the lens driving at that time is terminated, an out-of-focus situation of f.sub.r2 -l.sub.r2' =d.sub.x2' occurs.
As described above, in the aforedescribed method taking the release time-lag into account, the out-of-focus during state photographing becomes considerably small, but a certain extent of out-of-focus still occurs depending on the release timing.